Cu(I) and Cu(II) complexes of a pyridine-based pincer ligand

www.elsevier.com/locate/ica Inorganica Chimica Acta 330 (2002) 103– 110

Cu(I) and Cu(II) complexes of a pyridine-based pincer ligand Andrei N. Vedernikov, Peng Wu, John C. Huffman, Kenneth G. Caulton * Department of Chemistry and Molecular Structure Center, Indiana Uni6ersity, Bloomington, IN 47405 -7102, USA Received 16 July 2001; accepted 8 October 2001 Dedicated to the memory of Luigi M. Venanzi, a chemist and a gentleman

Abstract The pincer ligands 2,6-H3C5N(CH2NR2)2, LR, have been studied in their reaction towards CuCl2 and CuCl. For CuCl2, the case R= Et gives square-pyramidal (h3-LEt)CuCl2 with an apical CuCl distance 0.27 A, longer than the equatorial one. For R = iPr, the chloride-loss product (h3-LiPr)CuCl+ is established as its CuCl42 − salt. The mer geometry of the ligand in these two compounds is intolerable for Cu(I), and a ligand-redistribution product from CuCl is (h2-LMe)2Cu+, together with linear CuCl2− . Density functional theory (DFT) calculations of monomeric (LMe)Cu(I)Lq with L = MeCN, C2H4 or Cl− show a distinct tendency for one or both NMe2 arms to dissociate from Cu(I), while the Cu(II) analogs adopt planar geometry. © 2002 Published by Elsevier Science B.V. Keywords: Pyridine; Pincer ligand; DFT

1. Introduction Venanzi has pioneered the study of chelating amines and of polydentate ligands in general [1 – 6]. Much has been written about the impact of the very different coordination geometries and coordination numbers of Cu(I) and Cu(II) on their redox behavior, both in coordination and in (redox) metalloenzyme chemistry [7 – 9]. In brief, although Cu(II) can be four-, five-, or six-coordinate, Cu(I) can only be four-, three-, or twocoordinate. The intersection of these two, coordination number four, also represents a problem, since Cu(II) prefers a planar geometry and Cu(I) prefers a tetrahedral one. We report here a study of synthetic chemistry directed toward this question, using a pincer ligand based on an ortho disubstituted pyridine I.

* Corresponding author. Fax: + 1-812-855 8300. E-mail address: [email protected] (K.G. Caulton).

This would seem to be an obligatory mer ligand when it is tridentate on a metal, and thus to prevent a tetrahedral geometry. This work also shows the significant consequence of changing the four amine substituents from primary to secondary alkyl.

2. Experimental

2.1. General information All manipulations were carried out under purified Ar using standard Schlenk techniques. The synthesis of 2,6-di(tosyloxy-methyl)pyridine has been reported [10]. Solvents were distilled under Ar over appropriate agents, and stored in gas-tight solvent bulbs. CDCl3 was dried with CaH2 and vacuum-transferred prior to use. All the other reagents were used as purchased from Aldrich. 1H NMR spectra were recorded on a Varian Gemini 300 spectrometer (1H NMR, 300 MHz). 1H NMR chemical shifts are relative to TMS or use the residual solvent resonances as internal standard. Mass spectra were obtained on a Kratos MS80 RFAQQ instrument. m.p. was determined using a Thomas – Hoover capillary m.p. apparatus and are uncorrected.

0020-1693/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 8 2 5 - 8

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2.2. 2,6 -Bis(diethylaminomethyl)pyridine C15H27N3 2,6-Bis(tosyloxymethyl)pyridine (4.47 g, 10 mmol) and diethyl amine (30 ml, 290 mmol) were dissolved in C6H6 and heated to reflux for 10 h. After evaporation of the solvent, the residue was redissolved in 150 ml CH2Cl2, washed twice with saturated aq. NaHCO3 (100 ml), water (100 ml) and dried over Na2SO4. The solvent was removed and the product dried in vacuo. Yield: 2.20 g (88%). 1H NMR (25 °C, CDCl3): 7.59 (t, 1H), 7.32 (d, 2H), 3.69 (s, 4H), 2.55 (q, 8H), 1.02 (t, 12H).

2.3. 2,6 -Bis(diisopropylaminomethyl)pyridine C19H35N3 2,6-Bis(tosyloxymethyl)pyridine (4.47 g, 10 mmol) and diisopropylamine (30 ml, 214 mmol) were dissolved in C6H6 and heated to reflux for 10 h. After evaporation of the solvent, the residue was redissolved in 150 ml CH2Cl2, washed twice with saturated aq. NaHCO3 (100 ml), water (100 ml) and dried over Na2SO4. The solvent was removed and the product dried in vacuo. Yield: 2.00 g (66%). 1H NMR (25 °C, CDCl3): 7.55 (t, 1H), 7.42 (d, 2H), 3.74 (s, 4H), 3.03 (m, 4H), 1.01 (d, 24H).

2.4. Cu(2,6 -bis(diethylaminomethyl)pyridine)Cl2 (1) To an oven-dried 100 ml Schlenk flask containing a magnetic stirring bar was added, in a glove box, CuCl2 (134 mg, 1 mmol). To an oven-dried 50 ml Schlenk flask was added, in a glove box, 2,6-bis(diethylaminomethyl)pyridine (249 mg, 1 mmol). The flasks were fitted with serum caps, removed from the glove box, and the flask containing the ligand was charged with CH2Cl2 (20 ml); then the solution was added to the flask containing CuCl2 via cannula. The resulting bright green mixture was stirred rapidly for 2 h at room temperature (r.t.). The reaction mixture was reduced to half of its volume under reduced pressure and treated with C5H12. The green crystals were collected and washed with C5H12, then dried in vacuo. Yield: 368 mg (96%). The single crystal X-ray structure was determined for the green crystal obtained by layering of the resultant CH2Cl2 solution with C5H12. m.p. 146– 147 °C. FAB-MS: 347 (M −Cl). Anal. Calc. for C15H27Cl2CuN3: C, 46.94; H, 7.09; N, 10.95. Found: C, 46.35; H, 6.76; N, 11.44%.

2.5. [Cu(2,6 -bis(diisopropylaminomethyl)pyridine) Cl]2CuCl4 (2) To an oven-dried 100 ml Schlenk flask containing a magnetic stirring bar was added, in a glove box, CuCl2 (134 mg, 1 mmol). To an oven-dried 50 ml Schlenk flask was added, in a glove box, 2,6-bis(diisopropylaminomethyl)pyridine (305 mg, 1 mmol). The flasks

were fitted with serum caps, removed from the glove box, and the flask containing the ligand was charged with CH2Cl2 (20 ml), then the solution was added to the flask containing CuCl2 via cannula. The resulting dark green mixture was stirred rapidly for 2 h at r.t. The reaction mixture was taken to half of its volume under reduced pressure and treated with C5H12. The green crystals were collected and washed with C5H12, then dried in vacuo. Yield: 311 mg (92%). The single crystal X-ray structure was determined for the green crystal by layering of the resultant CH2Cl2 solution with C5H12. m.p. 144–145 °C. FAB-MS: 403 (Cu(2,6bis(diisopropylaminomethyl)pyridine)Cl+). Anal. Calc. for C38H70Cl6Cu3N3: C, 44.99; H, 6.96; N, 8.29. Found: C, 44.65; H, 6.48; N, 8.36%.

2.6. 2,6 -Bis(dimethylaminomethyl)pyridine 2,6-Bis(tosyloxymethyl)pyridine (4.47 g, 10 mmol) was added with stirring to an ice-cooled 2 M dimethylamine solution in thf (50 ml, 0.1 mmol). After dissolution of the tosylpyridine was complete, the mixture was allowed to warm slowly to r.t. and stirred overnight. Then finely powdered potassium hydroxide (2.6 g, 40 mmol) was added to the solution and the resulting mixture was stirred for 1 h at r.t. All solids were then filtered off, washed with CH2Cl2 and the filtrate evaporated. The residual oil was distilled in vacuum. Yield: 2.50 g (72%). b.p. 82–84 °C at 0.5 Torr. 1H NMR (25 °C, CDCl3): 2.46 (s, 12H), 3.76 (s, 4H), 7.45 (d, 7.7 Hz, 2H), 7.8 (t, 7.7 Hz, 1H).

2.7. Cu[(2,6 -bis(dimethylaminomethyl)pyridine]2CuCl2 (3) To a dry flask containing a magnetic stirring bar was added, in a glove box, CuCl (99 mg, 1.0 mmol) and 1.0 ml of dry C6H6. Then, to the stirred mixture 2,6-bis(dimethylaminomethyl)pyridine (193 mg, 1.0 mmol) was added. Stirring was continued at r.t. till complete dissolution of the solid (2 h). Evaporation of the resulting solution gave 292 mg (100%) of yellow oil, which crystallized slowly to give large yellow crystals suitable for X-ray structural determination. 1H NMR (25 °C, CDCl3): 2.21 (s, 12H), 3.64 (s, 4H), 7.0–7.4 (m, 3H).

2.8. X-ray structure determinations 2.8.1. Cu[H3C5N(CH2NEt2)2]Cl2 ·CH2Cl2 A nearly-equidimensional bright blue fragment cleaved from a larger crystal was affixed to a glass fiber using silicone grease and transferred to the goniostat where it was cooled to − 158 °C using a gas-flow cooling system of local design. Standard inert atmosphere techniques were used. The data were collected

A.N. Vederniko6 et al. / Inorganica Chimica Acta 330 (2002) 103–110

on a Bruker 6000 CCD diffractometer using five-second frames with an omega scan of 0.30°. Data were corrected for Lorentz and polarization effects and equivalent reflections averaged using the Bruker SAINT software as well as utility programs from the XTEL library. An absorption correction was performed using the SADABS program supplied by Bruker AXS. The structure was solved using SHELXTL and Fourier techniques. In addition to the Cu complex, a CH2Cl2 solvent molecule was located in the lattice. The solvent molecule exhibited a slight disorder in one of the two chlorine positions, but was readily modeled. A final difference Fourier was essentially featureless.

2.8.2. {Cu[H3C5N(CH2N iPr2)2]Cl}2CuCl4 A cleaved fragment of a larger crystal was affixed to a glass fiber using silicone grease and transferred to the goniostat where it was cooled to −158 °C using a gas-flow cooling system of local design. Inert atmosphere techniques were used during the examination and mounting of the crystal. The data were collected using 10-second frames with an omega scan of 0.30°. Data were corrected for Lorentz and polarization effects and equivalent reflections averaged using the Bruker SAINT software as well as utility programs from the XTEL library. An absorption correction was performed using the SADABS program supplied by Bruker AXS. The structure was solved using SHELXTL and Fourier techniques and consists of a CuCl4 anion lying on a twofold axis, and the cation lying in a general position. All hydrogen atoms were clearly located and refined isotropically in the final cycles of refinement. A final difference Fourier was essentially featureless with the largest peak being 0.23 e A, − 3.

2.8.3. Cu[H3C5N(CH2NMe2)2]2[CuCl2] A well-formed yellow, transparent, nearly-equidimensional fragment was cleaved from a larger crystal and transferred to the end of a glass fiber using silicone grease and cooled to 111 K for data collection. The data were collected using three-second frames with an omega scan of 0.30°. Data were corrected for Lorentz and polarization effects and equivalent reflections averaged using the Bruker SAINT software as well as utility programs from the XTEL library. The structure was solved using SHELXTL and Fourier techniques. Two independent cations and anions are present in the asymmetric unit. The absolute structure was determined by refinement of the Flack parameter in SHELX prior to final refinement using the XTEL library. All hydrogen atoms were located and refined. A final difference Fourier was essentially featureless with maximum peak heights of 0.42 A, − 3.

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2.9. Computational details Theoretical calculations were performed using the density functional theory (DFT) method [11], with the PBE functional [12], implemented in an original program package Priroda [13] and with the B3LYP functional [14], in program package GAUSSIAN 98 [15]. In the PBE calculations, relativistic Stevens–Basch– Krauss [16] effective core potentials (ECP) optimized for DFT calculations have been used. The basis set was 311-split for main group elements with one additional polarization p-function for hydrogen, and two additional polarization d-functions for elements of higher periods and 511/51/511 split for (n−1)sp/(n − 1)d/(n)sp shells of transition elements. In the B3LYP calculations, the SBK ECP and CEP31 basis set, which is a version of SBK, was used, the basis set being 31-split for non-transition elements and 411/421(p) split for (n− 1)d/(n)sp shells of transition metals. Full geometry optimization was performed without constraints on symmetry. For all species under investigation, frequency analysis was carried out. All minima were checked for the absence of imaginary frequencies. Geometries of some species obtained with the two theoretical methods are listed in Table S1 (Supplemental Information) to permit comparison of Priroda with GAUSSIAN 98. For Cu(II) complexes, the metal-donor atom bond lengths or donor atom-metal-donor atom bond angles usually differ less than 0.01 A, , and in rare cases by 0.04 A, and usually near 1° and in very rare cases by 2°. For Cu(I) complexes, the situation was the same, with the exception of cases when the metal was planar and four-coordinate. In these cases of unusual coordination geometry, differences in bond length were up to 0.2 A, , but these are for bonds that are so weak that ‘soft’ energy surfaces are to be expected. Calculated ligand dissociation energy variations for reactions 1–4, where bdmamp is 2,6-bis(dimethylaminomethyl)pyridine, were in the range 2.0–3.6, and only in rare cases, 9.0 kcal mol − 1. [Cu(bdmamp)]+ + MeCN “[Cu(bdmamp)(NCMe)]+ DE(PBE) = − 27.0 kcal mol − 1 DE(B3LYP) = − 24.8 kcal mol − 1

(1)

[Cu(bdmamp)]2 + + MeCN “[Cu(bdmamp)(NCMe)]2 + DE(PBE) = − 47.8 kcal mol − 1 DE(B3LYP) = − 51.4 kcal mol − 1

(2)

[Cu(bdmamp)]+ + C2H4 “ [Cu(bdmamp)(C2H4)]+ DE(PBE) = − 25.3 kcal mol − 1 DE(B3LYP) = − 16.3 kcal mol − 1

(3)

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[Cu(bdmamp)]2 + +C2H4 “[Cu(bdmamp)(C2H4)]2 + DE(PBE) = − 24.3 kcal mol − 1 DE(B3LYP) = − 22.3 kcal mol − 1

(4)

We can therefore conclude that for both Cu(II) and Cu(I) complexes of even unusual geometry, the geometrical parameters of the coordination unit and metal-ligand binding energies are in satisfactory agreement for the two DFT methods used here. Therefore, we can rely on results obtained in this paper with the PBE functional and the Priroda program. Fig. 1. ORTEP diagram of the non-hydrogen atoms, showing selected atom labeling in Cu(2,6-bis(diethylaminomethyl)pyridine)Cl2. Unlabeled atoms are carbon.

Table 1 Crystallographic data a 1 Empirical formula Formula weight Temperature (K) Color Space group a (A, ) b (A, ) c (A, ) i (°) V (A, 3) Z zcalc (g cm−3) u (A, ) v (cm−1) R Rw

2

3. Results

3

C16H29Cl4CuN3 C38H70Cl6Cu3N6 C22H38Cl2Cu2N4 468.78

1014.37

584.58

115

115

162

blue P21/n 10.7713(3) 16.2507(4) 13.1398(3) 110.557(1) 2153.55 4 1.446 0.71073 15.138 0.0249 0.0205

dark blue Fdd2 20.135(1) 56.083(2) 8.200(0)

yellow P21 10.285(1) 15.176(1) 17.685(1) 97.28(0) 2742.18 4 1.418 0.71073 17.666 0.0293 0.0277

9260.03 8 1.455 0.71073 17.455 0.0164 0.0172

a R = Fo − Fc / Fo ; Rw = [ w( Fo − Fc )2/ w Fo 2]1/2 where w = 1/| 2( Fo ).

Table 2 Bond distances (A, ) and angles (°) for [H3C5N(CH2NEt2)2]CuCl2·CH2Cl2 Bond lengths Cu(1)Cl(2) Cu(1)Cl(3) Cu(1)N(4) Cu(1)N(12) Cu(1)N(13)

2.506(1) 2.2354(8) 2.1703(26) 2.1222(26) 1.9387(24)

Bond angles Cl(2)Cu(1)N(4) Cl(2)Cu(1)N(12) Cl(2)Cu(1)N(13) Cl(3)Cu(1)N(4) Cl(3)Cu(1)N(12) Cl(3)Cu(1)N(13) N(4)Cu(1)N(12) N(4)Cu(1)N(13) N(12)Cu(1)N(13)

95.71(29) 104.37(28) 90.83(28) 98.49(7) 94.83(7) 167.10(8) 152.99(9) 80.20(10) 81.60(10)

3.1. NEt2 Pincer (1) This compound crystallizes from dichloromethane incorporating a molecule of CH2Cl2 in lattice sites that are not interacting with copper. The geometry (Tables 1 and 2 and Fig. 1) is square-pyramidal, the five-membered chelate rings are non-planar, and the pyridine nitrogen is over 0.18 A, closer to copper than is either NEt2 nitrogen. The apical chlorine forms a much longer bond (by 0.27 A, ) to Cu than does the equatorial Cl, consistent with the Jahn–Teller effect in Cu(II). The length of this bond makes it an easy target for removal by an electrophile (see below). The structure of [H3C5N(CH2NMe2)2]ZnCl2 was reported recently [17]. While it is square-pyramidal, like 1, the space group is not isomorphous to that of the Cu analog. The MN distances are 0.11–0.15 A, longer to Zn than to Cu, but a great difference between the two structures is that both ZnCl distances are 2.2790.01 A, (i.e. nearly identical). This supports the Jahn–Teller effect as the origin of the CuCl bond differences.

3.2. N iPr2 Pincer (2) The product that crystallizes from CH2Cl2 –pentane (Tables 1 and 3 and Fig. 2) is a salt with two (symmetry-related) monochloro cations and a CuCl42 − anion: [(pincer)CuCl]2[CuCl4]. In effect, two (pincer)CuCl2 molecules have each had one chloride abstracted by the electrophile CuCl2. The pyridine nitrogen has a much shorter CuN distance than those to NiPr2, and the latter are also more variable (i.e. by Z/|:10). The conformation of the two five-membered rings is non-planar, they are approximately related by a C2 axis, and this leaves the two iPr groups on a given N in either an axial or an equatorial site. The CuCl42 − anion (which has a crystallographic C2 axis) is somewhat distorted from planarity

A.N. Vederniko6 et al. / Inorganica Chimica Acta 330 (2002) 103–110 Table 3 Bond distances (A, ) and angles (°) for [H3C5N(CH2NiPr2)2CuCl]2(CuCl4) Bond lengths Cu(1)Cl(2) Cu(1)N(3) Cu(1)N(10) Cu(1)N(12) Cu(25)Cl(26) Cu(25)Cl(27)

2.1970(6) 2.1487(20) 1.9176(20) 2.1181(20) 2.2651(7) 2.2548(7)

Bond lengths Cl(2)Cu(1)N(3) Cl(2)Cu(1)N(10) Cl(2)Cu(1)N(12) N(3)Cu(1)N(10) N(3)Cu(1)N(12) N(10)Cu(1)N(12) Cl(26)Cu(25)Cl(26)% Cl(26)Cu(25)Cl(27) Cl(26)Cu(25)Cl(27)% Cl(27)Cu(25)Cl(27)%

97.46(6) 178.96(6) 98.41(6) 81.71(8) 164.01(8) 82.44(8) 92.68(4) 94.834(23) 146.370(24) 96.78(4)

Fig. 2. ORTEP diagram of [Cu(2,6-bis(diisopropylaminomethyl)pyridine)Cl]2CuCl4. Table 4 Bond distances (A, ) and angles (°) for [Cu{H3C5N(CH2NMe2)2}2][CuCl2] Ion A

Ion B

Bond lengths Cu(1)N(2) Cu(1)N(3) Cu(1)N(4) Cu(1)N(5) Cu(59)Cl(60) Cu(59)Cl(61)

1.983(3) 2.217(4) 2.025(3) 2.162(3) 2.1044(13) 2.1015(13)

1.990(3) 2.224(3) 2.026(3) 2.180(4) 2.0967(12) 2.0951(12)

Bond angles N(2)Cu(1)N(3) N(2)Cu(1)N(4) N(2)Cu(1)N(5) N(3)Cu(1)N(4) N(3)Cu(1)N(5) N(4)Cu(1)N(5) Cl(60)Cu(59)Cl(61)

81.93(14) 133.24(13) 135.03(13) 113.42(14) 113.73(14) 81.03(13) 178.96(6)

82.54(13) 134.69(13) 132.87(12) 116.38(12) 110.57(12) 81.23(12) 179.63(6)

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(the four cis angles sum to 375° compared to the ideal sums of 360° for planar and 438° for tetrahedral) and its CuCl distances are longer by 0.06 A, than that in the cation. The cation is quite planar (four angles sum to 360.0°), and the cis NCuN angles are small ( 82°). In general, all Culigand bond lengths are shorter by 0.02–0.04 A, in the cation than in (NEt2 pincer)CuCl2, an apparent consequence of the lower coordination number. It is noteworthy that the CuNiPr2 distances are not longer in the cation, in spite of the larger iPr groups. We suggest that the reason for different products when the amine substituent goes from a primary to a secondary alkyl is steric in nature: four bulky iPr groups encourage chloride loss to give a reduced copper coordination number.

3.3. [Cu(pincer)2]CuCl2 (3) The asymmetric unit contains two cations and two anions. The X-ray diffraction structure determination (Tables 1 and 4 and Fig. 3) reveals a linear two-coordinate anion showing no short contacts to a cation, while each pincer ligand is bidentate, through the pyridine N and one NMe2 arm; the other CH2NMe2 arm on each pincer ligand is not coordinated to Cu, nor does it interact with copper of the CuCl2− . The distance from these N to copper in the anion exceeds 8.1 A, . Distances from Cl to Cu in the cation exceed 5.3 A, . The two CuN2 planes involving different pincer ligands are orthogonal (dihedral angle between CuN2 planes is 89.9°), and the two CuN(py) distances are within 0.04 A, of each other, but the CuNMe2 distances differ by 0.055 A, . The longest CuN distance, to Me2N3, sets a coordination geometry which might be termed 3+ 1. This is supported by the fact that the angles involving Cu and N2, N4 or N5 sum to 349.3°, so copper approaches planarity with the three N to which it is most strongly bonded; N9 lies out of that plane. For comparison, the distance between Cu and the (non-bonded) N25 is 3.4 A, . The salt [Cu(Me2NCH2CH2NMe2)2][CuCl2] is an interesting comparison structure [18]. The linear CuCl2− anion has identical bond lengths to those reported here. The cation has two orthogonal CuN2 planes, the CuN distances there are less than 0.04 A, shorter than those to the NMe2 nitrogen here, and the intra-chelate ÚNCuN there is about 3° larger than that in the h2-pincer ligand here. Is there steric repulsion between the dangling CH2NMe2 arm and the second chelate ring? To investigate the hypothesis, DFT optimization was carried out on the full cation Cu[H3C5N(CH2NMe2)2]2+ . The optimized structure agrees with that found in the solid state to within 0.07 A, and 5°. A second DFT geometry optimization of the species Cu[H4C5N(CH2NMe2)]2+

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with the uncoordinated amine arm replaced by hydrogen gave essentially the same coordination geometry, with a dihedral angle between the two CuN2 planes of 86.3° (compare 86.2° calculated with pendant arms).

4. Computational studies

4.1. [H3C5N(CH2NMe2)2]Cu(Lewis base) n + Fig. 3. ORTEP diagram of one of the two independent cations Cu[2,6-bis(dimethylaminomethyl)pyridine]2+ .

Fig. 4. Calculated (DFT) minimum energy structures for [H3C5N(CH2NMe2)2]Cu(I)L for L =MeCN (top), C2H4 (middle) and Cl− (bottom)O. Hydrogens have been omitted, for clarity.

Scheme 1.

4.1.1. Mono6alent copper Geometries where Lewis base is MeCN, ethylene, and chloride were all optimized without symmetry constraints (Fig. 4) using the DFT (PBE) method. For Cu(I), all showed a tendency of one of the two NMe2 arms of ligand L to dissociate from copper (II). Thus, while CuN(py) optimized in the range 1.95–2.02 A, ,

and one CuNMe2 distance of 2.08–2.46 A, , the second CuNMe2 distance lengthened to 2.65–3.59 A, ; this latter we interpret as the result of attempts to fully dissociate this nitrogen from copper. All of these are symptomatic of four-coordinate Cu(I) not accepting a planar geometry and instead moving towards a threecoordinate form. This interpretation is further strengthened by the angle a, which optimizes to 108.2°, 128.8°, and 110.1°, when L is NCMe, C2H4 and Cl−. Along this series, angle b is relatively constant (79.2– 85.1°). Why is the arm of the chelate dissociated, and not the ligand L? This is because the former product can approach a preferred trigonal three-coordinate geometry rather than the latter (T-shaped, with no compensation at the vacated coordination site). Finally, why is this (four-coordinate) 18-valence electron species not preferred to the 16-electron one? It is because the last electron added (cf. the Cu(II) analog discussed below) completes the double occupancy of the dx 2 − y 2 orbital (III), which is CuN antibonding, but only half-filled for Cu(II).

To test the validity of these conclusions in the absence of chelate ring constraints, calculations were performed on Cu(NH3)4+ . These reveal (Scheme 1, energies in kcal mol − 1) that a square-planar geometry

A.N. Vederniko6 et al. / Inorganica Chimica Acta 330 (2002) 103–110

is hugely destabilized relative to tetrahedral geometry, and that even loss of one NH3, to give a threefold symmetric planar Cu(NH3)3+ is more stable than the square-planar structure. In these calculations, all structures except IV and V are stationary points. Structure V has been chosen as a model of a square-planar Cu(I) complex with extremely rigid ligand skeleton, so all CuN bond lengths have been constrained. Remarkably, the ligand loss process is also a very effective way to stabilize from the square-planar geometry: although the stabilization is not as good as going to a tetrahedral structure, the relaxation of the T-shape Cu(NH3)3+ geometry to trigonal is so important that the dissociation products are only 11.6 kcal mol − 1 above the intact tetrahedron. Thus, the bond strength of removing the first NH3 from tetrahedral Cu(NH3)4+ is only 11.6 kcal mol − 1, and the dissociation of one arm from square-planar Cu(I) observed here experimentally is thermodynamically very favorable.

Fig. 5. Calculated (DFT) [H3C5N(CH2NMe2)2]Cu+.

minimum

energy

structure

for

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A calculation was also done for (pincer)Cu+, to evaluate whether this could be a minimum, and if so, the hapticity of the pincer ligand. In fact, this species minimizes to the geometry in Fig. 5 showing near C2 symmetry (axial and equatorial methyls) and with the CuN(py) distance shorter than those to NMe2, but with the CuNMe2 distance the shortest of any species calculated here. Copper(I) thus accepts coordination number three, and the geometry is probably as near to planar around Cu (NCuN angles sum to 329.6°) as is possible, given the constraints of two five-membered rings fused to a six-membered ring. It is instructive to compare the calculated structures of three-coordinate (pincer)Cu+ with that of its MeCN adduct (Fig. 1). While the four-coordinate MeCN adduct shows CuNMe2 distances which are long enough to be on the verge of fully dissociating from copper (2.38 and 2.65 A, ), these distances become fully bonding (2.07 A, ) in (pincer)Cu+. Coordination number three is thus preferred for (pincer)Cu+, even if the geometry is Tshaped, rather than trigonal. In contrast, in both species, the CuN(py) distance differs insignificantly (0.04 A, ).

4.1.2. Di6alent copper The Cu(II) analogs of the above three species show dramatic geometric consequences of removal of only one electron. All show equal CuN distances to the NMe2 arms, and thus the antibonding influence in the Cu(I) analogs is much diminished. Certainly, the CuNMe2 distances, 2.15–2.20 A, , are longer than those to pyridine N (1.93–1.95 A, ), but there is no tendency for any ligand loss. In every case, L is closer to Cu(I) than to Cu(II), but this is in part due to the smaller coordination number of Cu(I). The CuL lengthening is 0.08–0.11 A, for the nitrogen L, but 0.23 A, for ethylene, which is consistent with the general absence of olefin complexes of di6alent copper (Fig. 6).

5. Conclusions

Fig. 6. Calculated (DFT) minimum energy structures for [H3C5N(CH2NMe2)2]Cu(II)L for L = MeCN (top), C2H4 (middle) and Cl− (bottom). Hydrogens have been omitted, for clarity.

Density functional calculations nicely model the experimental structural features in the situations studied here, where the pincer ligands on Cu(I) present a stereoelectronic challenge. In particular, DFT calculations on a single molecule show efforts, in the geometry optimization, to go to lower energy states by detaching one ligand lone pair. Coordination number three, or even two, is preferable to a planar four-coordinate structure. The experimental situation for ‘(pincer)CuCl’ is more complex than that modeled by a single molecule, since the real system reveals a ligand redistribution. Two pincer ligands bind to one Cu+, and the second CuCl accepts a second chloride. Each pincer ligand is only bidentate, through one imine and one

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amine N, and the CuN4+ geometry approaches tetrahedral. It is thus better to have one two-coordinate (i.e. unsaturated) Cu(I) than to have two planar four-coordinate Cu(I). As a result of the ‘two molecule’ (or two empirical formula unit) redistribution, the DFT calculations we carried out could not have gone to the experimental result. The fact that the DFT calculations did give CH2NR2 ‘arm’ loss was, however, an indication of proper response to an intolerable (planar) structure, and indeed may represent the mechanism for the observed formation of [Cu(pincer)2]CuCl2. In general, although examples of C6H3N(CH2NR2)2 being only bidentate are unknown, the corresponding h2 case with aryl as the linker (VI) is well established [19,20].

6. Supplemental information A comparison of DFT calculated structures as described in Section 2.

Acknowledgements A.N.V. is grateful to Professor Kenneth G. Caulton for a postdoctoral fellowship at Indiana University. This work was supported by the U.S. Department of Energy.

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